The adverse environmental effects of chlorofluorocarbons as propellants, which have been in widespread use in pressurized aerosol dispensers, have led to their replacement by hydrocarbons, carbon dioxide and other non-miscible or not readily miscible propellants. The hydrocarbons, which are most commonly used, are highly flammable and would be dangerously volatile if combined with the solvents formerly used in aerosol mixtures intended for use with fluorocarbon propellants.
To avoid dangerous volatility the aerosol product solutions have been changed to water soluble formulations. The aerosols have a three layered formation in the container because the hydrocarbon propellant, a paraffin derivative, is not miscible with the water based solution. In the container the water based product solution is at the bottom. Floating on top of the product solution is a non-miscible hydrocarbon layer in a liquid state. Above this liquid hydrocarbon layer the remainder of the container is occupied by the hydrocarbon propellant in its gaseous state.
The product is dispensed from the container by opening a dispensing valve in the usual manner, with the gaseous pressure of the hydrocarbon propellant forcing the product up through the usual eduction tube and through the dispensing valve to the spray nozzle. As the product is dispensed from the container, the liquid hydrocarbon layer on top vaporizes thereby maintaining an adequate propellant gas pressure inside the container.
With non-miscible or partially miscible propellants, such as the hydrocarbons substituted for the previous propellants, the aerosol dispensers of the type formerly in widespread use tended to produce a product spray which was too coarse and contained irregular sized droplets. Some improvement is achieved by providing a vapor tap in the valve housing to permit some of the gaseous propellant to enter the stream of water-based product for breaking up the product droplets as they are sprayed. However, this expediant has not been entirely effective in that the product spray still is coarse compared to the spray achieved with the fluorocarbon propellants. Also, if the user shakes the container before or during use, some liquid hydrocarbons may enter the vapor tap and be spit out of the discharge nozzle, presenting a serious safety hazard because of flammability. Also, shaking may cause small particles of foreign matter to enter and clog the vapor tap or orifices thus interfering with its effectiveness. A proposal to alleviate these problems is described in "Aerosol Age," July, 1977 pages 18-21, the article entitled "Precision unveils hydrocarbon water-based Aquasol system."